1. Field of the Invention
The present invention relates to a mobile communication terminal for use in a cellular mobile communication network system.
2. Description of the Related Art
In a cellular mobile communication network system, a plurality of base stations are provided in a service area, and each base station forms a radio zone called a cell. In each cell, the corresponding base station is connected by radio to mobile communication terminals. In this system, when a mobile communication terminal is turned on, synchronization is established between the terminal and the closest base station. After the establishment of synchronization, the terminal shifts to a standby state. In the standby state, an incoming call is transmitted from the base station to the terminal, or negotiation is performed for establishing a communication link between the terminal and base station if the terminal performs an operation for an outgoing call. When the communication link is established, the terminal shifts to a communication state.
In a cellular mobile communication network system, hand-off is performed when a mobile communication terminal moves from a cell to another cell. “Hand-off” means a process for changing, to another, the base station as a synchronization establishment destination. In the standby state and communication state, a mobile communication terminal acquires, from the base station with which synchronization is established (hereinafter referred to as an “active base station”), a neighbor list message listing the base stations located near the active base station. The mobile communication terminal searches for the neighboring base stations, using this message, and performs hand-off processing, if necessary.
For example, in a system employing a code division multiple access (CDMA) method, in which a mobile communication terminal establishes synchronization with a base station using sliding correlation therebetween, each base station transmits a pilot signal diffused by a diffusion code. The diffusion code used to diffuse the pilot signal is provided with an initial phase offset value, which varies between base stations. Each mobile communication terminal receives pilot signals transmitted from peripheral base stations, using the offset values recited in the neighbor list message, thereby measuring the reception quality of the pilot signals. The measurement of reception quality is performed by, for example, detecting the signal/interference-wave ratio (SIR) or reception-signal level. Based on the measurement results, the mobile communication terminal selects a peripheral base station of the best reception quality, and uses this base station as a destination candidate with which synchronization is to be established.
On the other hand, if all the measured reception quality levels are lower than a reference value, the mobile communication terminal determines that there is no appropriate base station for a destination candidate. In other words, it is determined that all peripheral base stations are out of range. After that, the terminal searches the base stations over all phases of the diffusion code used to diffuse pilot signals, and selects the base station from which the best reception quality can be acquired. Searching over all phases of a diffusion code corresponds to initial resetting for selecting a base station upon the turn-on of the mobile communication terminal. Accordingly, this operation requires a lot of time and power consumption. If it is performed frequently, the battery life of the terminal is shortened.
For various reasons, “Out of Range” determination is made. For example, the acquired neighbor list may become an inappropriate one due to a change in the positional relationship between the mobile communication terminal and base stations.
FIG. 4 is a view useful in explaining such a case. In the figure, assume that a mobile communication terminal MS establishes synchronization with a base station BS-A, and the neighbor list acquired from the active base station BS-A lists a peripheral base station BS-B. In this state, even if the terminal MS moves to a second position, the communication state does not change. However, if the terminal MS further moves to a third position, the communication between the terminal and base station BS-A is interrupted by an obstacle 1. At this time, the terminal MS hands off from the base station BS-A to the peripheral base station BS-B listed in the neighbor list.
After that, if the terminal MS moves to a fourth position, communication with the base station BS-B is interrupted by an obstacle 2. The fourth position is close to a base station BS-C. However, if the neighbor list acquired from the base station BS-B does not list the base station BS-C, and a pilot signal of sufficient reception quality cannot be acquired from any other peripheral base station listed in the neighbor list, the terminal MS determines that it is out of the range of the base stations listed in the neighbor list. In this case, the mobile communication terminal MS must perform the above-mentioned searching over all phases of a diffusion code.
Since the transmission lines for mobile communication are very complex, it is difficult to prepare a neighbor list that enables optimal hand-off processing to be executed in all positions in its service area. Therefore, it is a frequent practice to perform searching over all phases of a diffusion code.
There are known techniques for reducing the power consumption of a mobile communication terminal by shortening the time required for base-station searching for hand-off. Jpn. Pat. Appln. KOKAI Publication No. 2002-171555, for example, discloses the following technique (pages 2 to 5 and FIG. 1): When a mobile communication terminal selects a peripheral base station as a hand-off destination candidate, it stores, in its database, information indicating the base station, together with positional information indicating the position of the terminal itself. When the terminal again enters the cell indicated by the positional information, it selects the information, stored in the database, in preference to a neighbor list acquired from a base station corresponding to the cell. Based on the selected information, the terminal searches the peripheral base stations for a hand-off destination candidate. This search technique, however, requires a function for acquiring information indicating the position of a mobile communication terminal, which increases the circuit scale of the terminal and the amount of processing by the CPU of the terminal.
Further, Jpn. Pat. Appln. KOKAI Publication No. 2001-54156 discloses a technique for searching for a pilot signal transmitted from a peripheral base station, acquiring a neighbor list from the base station detected by searching, and selecting a hand-off destination candidate from the acquired neighbor list. In this technique, to acquire a neighbor list from a peripheral base station, a mobile communication terminal holds synchronization with the active base station, while periodically searching peripheral base stations other than the active one to select an accessible base station. After selecting the accessible base station, the terminal must perform processing for acquiring a neighbor list therefrom. This processing significantly reduces the battery life of the terminal.